In recent years, there has been a demand for a further improvement on energy saving in a working machine such as a hydraulic excavator, a wheel loader or the like. Here, for the energy saving of the working machine, the energy saving of a hydraulic system itself becomes important. Therefore, there have been made studies on adopting a hydraulic closed circuit system, in which a hydraulic pump and a hydraulic actuator are connected in a closed circuit and the hydraulic actuator is directly controlled by the hydraulic pump, for the working machine (Patent Document 1).
The hydraulic closed circuit system can control pressure losses due to a control valve to be smaller as compared to a hydraulic open circuit system that connects a hydraulic pump and a hydraulic actuator in an open circuit. Along with it, since the hydraulic pump delivers only a required flow amount, it is possible to suppress the flow amount loss also. In addition, it is possible to perform regeneration of potential energy or kinetic energy at deceleration of the actuator. Accordingly, the high-level energy saving is made possible.
Here, Patent Document 1 describes the working machine using the hydraulic closed circuit with a hydraulic drive system in which when a hydraulic cylinder is at a stop, the delivery pressure of a charge pump is made lower than usual, thus making it possible to reduce the loss of consumption power in the charge pump.